The present invention relates to the vacuum tube arts. It finds particular application in conjunction with high power, rotating anode x-ray tubes and will be described with particular reference thereto. It is to be appreciated, however, that the invention will find application in conjunction with other types of x-ray tubes and tubes in which high temperature target operation causes a carbon monoxide outgassing problems.
Heretofore, x-ray tubes have included an evacuated envelope which held a cathode and an anode. The anode included a composite target with tungsten tracks into a backing material. Electrons emitted by a cathode filament were drawn to a target area of the anode by a high voltage. The impact of the electron beam on the anode target causes high heating and the emission of x-rays. To dissipate the heat, means were provided for rotating the anode. As the anode rotated, each spot on the tungsten track that was heated by the electron beam rotated about 360.degree. before again receiving the electron beam. This worked well for low dissipation targets, particularly at temperatures below 1000.degree. C. However, as target temperatures were increased into the range of 1100.degree.-1400.degree. C. for higher performance, additional measures were required to prevent thermal damage.
To increase thermal power dissipation, the anode bodies were partially coated with a thermally emissive oxide layer. Typical oxides include aluminum titania oxide, in which the titanium dioxide is oxygen deficient resulting in very black coating.
Although the oxides are effective for dissipating the heat energy, the small amount of carbon in the titanium zirconium molybdenum (TZM) composite anode body tends to migrate to the surface, reacting with the oxide and forming carbon monoxide gas. The escape of carbon monoxide into the vacuum space of the tube destroys the vacuum. Although the anode composite typically contains only about 100 parts per million of carbon, when heated to the 1100.degree.-1400.degree. C. range, sufficient carbon monoxide is generated to reduce tube life through vacuum degradation. Even with the fastest gettering available with current technology, the carbon dioxide pressure becomes sufficiently high that it causes instability, sputtering of materials, crazing and even puncture of the glass envelope.
One proposed solution was to apply a 20-80 zirconium molybdenum alloy with a low pressure plasma spray to the anode body before applying the oxide coating. The plasma sprayed alloy contained about 15%-20% zirconium and 80%-85% molybdenum. Although this layer appears to reduce carbon monoxide emissions when the tube is new, it quickly becomes ineffective. The rate of carbon monoxide emission soon becomes the same with the plasma sprayed zirconium molybdenum alloy layer as without.
The present invention contemplates a new and improved anode construction which overcomes the abovereferenced problems and others.